The giant water waves which rise and fall periodically in the sea are called tides. Sometimes these tides are so high that they appear like high walls moving on the surface of the sea. Do you know what causes these tides? The main cause behind the formation of these tides in the sea is the force of attraction between the Earth and the moon. We know that all the bodies in the Universe attract one another with a force known as gravitational force. This force also acts between the Earth and the moon. The moon is pulled towards the Earth and the Earth towards the moon. This force of gravitation acting between the Earth and the moon is so strong that it can easily break a steel rod with a diameter of 400 kilometres. This force affects the solid part of the Earth slightly, but it causes rise and fall in the seawater noticeably. At any point in the sea, the water level keeps on rising for 6 hours and subsequently for next 6 hours, it keeps on falling. This rise and fall of the water level is known as tide. The water level rises after every 12 hours 25 minutes. This means that during one day and one night, i.e., 24 hours and 50 minutes, every place in the sea experiences tides two times. Tides are formed both in the seas facing the moon as well as those on the other side. Since the moon comes to the opposite side of the Earth after 12 hours and 25 minutes, tides are again formed on the same two spots. The force of attraction between the Sun and the Earth also affects the formation of tides but to a very small extent. When the Sun, Earth and moon come in a straight line, the Earth experiences the maximum force of attraction. And the tide caused on such occasions is very large. It is known as the ‘spring tide’. The sea experiences such tides on the full moon and the new moon days. When the Sun and moon are in a perpendicular direction, the Earth experiences the minimum force of gravitation. The tide formed on such occasions is very small and is called the ‘neap tide’. Such tides occur between the full moon and the new moon days. Tides can be as high as 15 metres. Tides are useful in many ways. They bring many precious items of sea, on to the shore. The tidal power of the sea is also being used to generate electricity. n How is the depth of the sea measured? Before the early part of the 20th century, the only known method of determining the depth of the sea at any point was by means of a cable. The ship stopped and a hemp line or wire with a weight at the end was lowered to the bottom. This was a slow, laborious task and not very accurate. Today man has developed innumerable instruments that help him to go deep into the seas and acquire more and more knowledge about their bottoms. Do you know, how the depth of the sea is measured with the help of modern instruments? The equipment which is used to measure the depth of the sea is called a ‘fathometer’. It is installed on a ship. It produces sound waves having frequencies of more than 20,000 hertz. These are called ultrasonic waves. They cannot be heard by human ears. These waves are transmitted inside the sea. They are reflected back by the bottom of the sea. The reflected waves are received by a receiver and the total time taken by them to travel from the sea surface to the bottom and back to the surface is measured. Half of this time multiplied by the velocity of sound in seawater gives the depth of the sea at the point of the experiment. Thus, the depth of the sea can be measured at any point. This technique is known as echo sounding or echo ranging. This technique has been used to measure the depths of various seas and the deepest spot in each of the seas has been located using this technique. The Pacific Ocean is the deepest ocean. The average depth of this ocean is 4,282 metres (14,048 feet). The deepest spot in this ocean is located near Guam and has a depth of 10,668 metres (35,000 feet). Guam is an island in the Pacific Ocean. It is about 5,000 kilometres (300 miles) to the west of the Hawaii islands. A more recent survey located a spot with a depth of 10,924 metres (35,839 feet) — as per the data obtained by Japan Maritime Safety Agency, using a narrow multi-beam echo sounder. Using depth as a criterion, the Indian Ocean occupies the second place. Its average depth is 3963 metres (13,002 feet). The Atlantic Ocean, coming third, has an average depth of 3926 metres (123,880 feet). Amongst the shallow seas, the Baltic Sea occupies the first place. Its average depth is 55 metres (180 feet). Studies of the oceanic crust have revealed that they are also as uneven as the surface of the Earth. They are full of the skeletons of the aquatic animals as well as big and small sea plants. Volcanic ash is also found there. The most modern method of studying the oceanic crust is to use the laser beams. The laser beams are transmitted into the water of the ocean and the time taken by them from the surface to the bottom and back is measured. On multiplying the velocity of speed of light with this time, one gets twice the distance of the bottom of the sea. This method is very accurate.